Laboratory and pilot-scale dead-end ultrafiltration concentration of sanitizer-free and chlorinated lettuce wash water for improved detection of Escherichia coli O157:H7

Potential of Flow Cytometric Approaches for Rapid Microbial Detection and Characterization in the Food Industry-A Review

As microbial contamination is persistent within the food and bioindustries and foodborne infections are still a significant cause of death, the detection, monitoring, and characterization of pathogens and spoilage microorganisms are of great importance. However, the current methods do not meet all relevant criteria. They either show (i) inadequate sensitivity, rapidity, and effectiveness; (ii) a high workload and time requirement; or (iii) difficulties in differentiating between viable and non-viable cells. Flow cytometry (FCM) represents an approach to overcome such limitations. Thus, this comprehensive literature review focuses on the potential of FCM and fluorescence in situ hybridization (FISH) for food and bioindustry applications. First, the principles of FCM and FISH and basic staining methods are discussed, and critical areas for microbial contamination, including abiotic and biotic surfaces, water, and air, are characterized. State-of-the-art non-specific FCM and specific FISH approaches are described, and their limitations are highlighted. One such limitation is the use of toxic and mutagenic fluorochromes and probes. Alternative staining and hybridization approaches are presented, along with other strategies to overcome the current challenges. Further research needs are outlined in order to make FCM and FISH even more suitable monitoring and detection tools for food quality and safety and environmental and clinical approaches.

Influence of product volume on water antimicrobial efficacy and cross-contamination during retail batch washing of lettuce

Fresh produce shall be thoroughly washed at the retail level prior to serving to the consumer with potable water. Foodborne pathogens if present on a product may transfer to the wash water and cross-contaminate other products immersed in the water. Typically, an entire carton of lettuce (24 heads) is washed together increasing the likelihood of cross-contamination due to the close contact between each head. This study aimed to compare the effects of two wash batch volumes - "low" (8 heads) and "high" (24 heads) on the efficacy of two commercial water antimicrobials and cross-contamination. Red leaf lettuce was spot-inoculated (~5.0 log CFU/g) with Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes. In the first batch of washing, inoculated lettuce was washed with non-inoculated lettuce, followed by reuse of the water/antimicrobials twice washing only non-inoculated lettuce. Samples of inoculated and non-inoculated lettuce were collected to determine aerobic plate count (APC) as well as the populations of STEC and L. monocytogenes. Microbiological analysis of the wash water was also conducted. Wash volume (8 versus 24 lettuce heads) had limited effects on the antimicrobial activities of the interventions evaluated. Instead, high-volume wash increased the rate of cross-contamination between non-inoculated and inoculated lettuce, and cross-contamination of non-inoculated lettuce through wash water reuse. Retailers should consider volume of product processed per batch, reuse of wash water, and use of an appropriate water antimicrobial in consideration of mitigating potential product cross-contamination.

Suitability of centrifuge water for detecting the presence of Escherichia coli versus finished fresh-cut lettuce testing

Fresh produce causes most foodborne outbreaks in the USA, and it is also considered a hazardous food product in other areas of the world such as Europe. The outbreaks attributed to fresh produce increase the focus of producers on hygiene to minimize exposure to food hazards. The fresh produce industry has the urgent need to detect if there are production lots contaminated with pathogenic microorganisms before distribution. Although the industry is mostly using end-product testing for the detection of target microorganisms, previous studies have evaluated the suitability of different sampling points within the production line of a fresh-cut processing plant. In the present study, the centrifuge effluent water was assessed as an alternative sampling point to end-product testing. E. coli was selected as an index microorganism of the presence of pathogens. The presence of E. coli was assessed in centrifuge effluent water, and fresh-cut lettuce from a commercial fresh-cut produce processing line (n = 95). The rate of false positives and negatives, as well as the specificity, sensitivity, and efficiency of the alternative method were calculated. The mean population of E. coli in positive water samples was 0.86 log cfu/100 mL, while the mean population of E. coli in positive fresh-cut lettuce samples was 0.23 log cfu/g. The proportion of positive samples in centrifuge effluent water and lettuce was similar (≈20%), and most of the results in both matrices were coincident (81.1%). However, the alternative method was not reliable due to its low sensitivity, as only 47.6% of the lettuce samples positive for E. coli could be matched with positive water samples.

Transfer and Redistribution of Salmonella Typhimurium LT2 and Escherichia coli O157:H7 during Pilot-Scale Processing of Baby Spinach, Cilantro, and Romaine Lettuce

Several outbreaks of foodborne illness traced to leafy greens and culinary herbs have been hypothesized to involve cross-contamination during washing and processing. This study aimed to assess the redistribution of Salmonella Typhimurium LT2 during pilot-scale production of baby spinach and cilantro and redistribution of Escherichia coli O157:H7 during pilot-scale production of romaine lettuce. Four inoculated surrogate:uninoculated product weight ratios (10:100, 5:100, 1:100, and 0.5:100) and three inoculation levels (103, 101, and 10-1 CFU/g) were used for the three commodities. For each of three trials per condition, 5-kg batches containing uninoculated product and spot-inoculated surrogate products at each ratio and inoculation level were washed for 90 s in a 3.6-m-long flume tank through which 890 L of sanitizer-free, filtered tap water was circulated. After washing and removing the inoculated surrogate products, washed product (∼23, 225-g samples per trial) was analyzed for presence or absence of Salmonella Typhimurium or E. coli O157:H7 by using the GeneQuence Assay. For baby spinach, cilantro, and romaine lettuce, no significant differences ( P > 0.05) in the percentage of positive samples were observed at the same inoculation level and inoculated:uninoculated weight ratio. For each pathogen product evaluated (triplicate trials), inoculation level had a significant impact on the percentage of positive samples after processing, with the percentage of positive samples decreasing, as the initial surrogate inoculation level decreased. The weight ratio of contaminated:noncontaminated product plays an important role: positive samples ranged from 0% to 11.6% ± 2.05% and from 68.1% ± 33.6% to 100% among the four ratios at inoculation of 10-1 and 101 CFU/g, respectively. To our knowledge, this study is the first to assess the redistribution of low levels of pathogens from incoming product to leafy greens during processing and should provide important data for microbial risk assessments and other types of food safety analyses related to fresh-cut leafy greens.

Persistence of Escherichia coli O157:H7 during pilot-scale processing of iceberg lettuce using flume water containing peroxyacetic acid-based sanitizers and various organic loads

In order to minimize cross-contamination during leafy green processing, chemical sanitizers are routinely added to the wash water. This study assessed the efficacy of peroxyacetic acid and mixed peracid against E. coli O157:H7 on iceberg lettuce, in wash water, and on equipment during simulated commercial production in a pilot-scale processing line using flume water containing various organic loads. Iceberg lettuce (5.4kg) inoculated to contain 106CFU/g of a 4-strain cocktail of non-toxigenic, GFP-labeled, ampicillin-resistant E. coli O157:H7, was shredded using a commercial shredder, step-conveyed to a flume tank, washed for 90s using water alone or two different sanitizing treatments (50ppm peroxyacetic acid or mixed peracid) in water containing organic loads of 0, 2.5, 5 or 10% (w/v) blended iceberg lettuce, and then dried using a shaker table and centrifugal dryer. Thereafter, three 5.4-kg batches of uninoculated iceberg lettuce were identically processed. Various product (25g) and water (50ml) samples collected during processing along with equipment surface samples (100cm2) from the flume tank, shaker table and centrifugal dryer were then assessed for numbers of E. coli O157:H7. Organic load rarely impacted (P>0.05) the efficacy of either peroxyacetic acid or mixed peracid, with typical reductions of >5logCFU/ml in wash water throughout processing for all organic loads. Increases in organic load in the wash water corresponded to changes in total solids, chemical oxygen demand, turbidity, maximum filterable volume, and oxidation/reduction potential. After 90s of exposure to flume water, E. coli O157:H7 reductions on inoculated lettuce ranged from 0.97 to 1.74logCFU/g using peroxyacetic acid, with an average reduction of 1.35logCFU/g for mixed peracid. E. coli O157:H7 persisted on all previously uninoculated lettuce following the inoculated batch, emphasizing the need for improved intervention strategies that can better ensure end-product safety.